1. Field of the Invention
This invention relates to a process of producing concentrated sulfuric acid by a catalytic reaction of SO.sub.2 to SO.sub.3, wherein SO.sub.2 -containing, hot, humid gases are used in which the ratio of H.sub.2 O to SO.sub.3 is sub-stoichiometric, and the catalytic reaction of the SO.sub.2 content is effected in two contacting stages, the reactant gas is partly reacted in the first contacting stage and is then cooled by an indirect heat exchange, the cooled reactant gas is cooled further in that concentrated sulfuric acid is sprayed into the gas, such as in several steps, whereby sulfuric acid vapor is condensed and residual SO.sub.3 and water vapor are absorbed, and the gas is subsequently heated by an indirect heat exchange and is then fed to the second contacting stage.
2. Discussion of Prior Art
The catalytic reaction of SO.sub.2 to SO.sub.3 in contacting plants may be carried out with dried gases or by so-called wet catalysis with humid gases. Humid gases contain a water content which is sufficient to form sulfuric acid with at least part of the formed SO.sub.3 by condensation. The use of humid gases affords the advantage that drying costs can be saved. On the other hand, sulfuric acid mist may be formed as the H.sub.2 O and SO.sub.3 contents of the gases condense.
It has also been proposed to carry out the catalytic reaction in two contacting stages and to remove by an intermediate condensing and absorbing step the SO.sub.3 which has been formed by the reaction in the first contacting stage, as well as the water vapor content. The residual SO.sub.2 is then substantially reacted to form SO.sub.3 in the second contacting stage, in which a dry catalysis is effected, and the SO.sub.3 formed in said second stage is absorbed in an end absorber, which succeeds the last contacting tray. In this way, the drying stage required for a catalysis with dry gases can be saved. Such process is known from British patent specification No. 475,120. The gases leaving the first contacting stage contain water in a quantity which is stoichiometrically related to the SO.sub.3 content of the gases for a condensation of 90 to 98% sulfuric acid and are slowly cooled to about 200.degree. to 250.degree. C. by an indirect heat exchange. A substoichiometric ratio of H.sub.2 O to SO.sub.3 will cause a formation of finely dispersed mists, which can be removed only with great difficulty. The disadvantages reside in that it is not possible under the conditions of practical operation to adjust and maintain the stoichiometric water vapor contents by an exactly proportioned addition of water vapor so that an optimum condensation cannot be effected. Additionally, the sulfuric acid vapor pressure under the operating conditions of the condensing system is so high that considerable mist problems arise in the end absorber.
It is known from U.S. Pat. No. 2,471,072 to cool the gases which have left the first contacting stage at a temperature of 538.degree. to 704.degree. C. to a temperature below 166.degree. C. by an injection of counter-current sulfuric acid having a concentration up to 93%. This results in a formation of sulfuric acid having a concentration of 60 to 93%. In a second stage, the gases are cooled by an indirect heat exchange to 24.degree. to 43.degree. C. This results in a formation of sulfuric acid having a concentration of 4 to 6%. Thereafter, the gases are passed through a coke filter and a drying tower and subsequently enter the second contacting stage. The disadvantages reside in that weak sulfuric acid is produced and an additional drying stage is required after the condensation.
From German No. 2,725,432 it is known to inject counter-current sulfuric acid into the uncooled gas which has left the first contacting stage. This results in a formation of sulfuric acid having a concentration of 96 to 98%. The ratio of H.sub.2 O to SO.sub.2 in the gas which is used amounts to 1.0 to 1.25, which means that when more than 90% of the SO.sub.2 are reacted in the first contacting stage the ratio of H.sub.2 O to SO.sub.3 will be stoichiometric for the condensation of acid containing 95 to 99% by weight of H.sub.2 SO.sub.4. The stoichiometric ratio of H.sub.2 O to SO.sub.3 for a formation of sulfuric acid having a concentration of 95 to 99% has the disadvantage that the dew point temperature of the gases is very high so that the valuable sensible heat of the gas cannot be used, for instance, to generate steam. For this reason the entire sensible heat of the gases leaving the first contacting stage as well as the heat of condensation must be dissipated in the acid cooler of the acid cycle of the condensing tower, and the entire surplus gas heat of the contacting system is required to reheat the cold gases leaving the condensing system. A sub-stoichiometric operation would result in a very strong formation of mist in the condensing tower.
It is known from U.S. Pat. No. 4,064,223 to cool the gas leaving the first contacting stage to a temperature of 180.degree. to 220.degree. C. by an indirect heat exchange and then to inject cold sulfuric acid having a concentration of 96 to 99% into the gas cocurrently in a first cooling stage and counter-currently in a second cooling stage, whereby the gas is cooled to 40.degree. to 60.degree. C. and the water vapor is removed. The gas is subsequently heated to 200.degree. C. and counter-current cold sulfuric acid having a concentration of 98 to 99% is injected into the gas in an absorber, to effect an absorption of SO.sub.3. The gas is then reheated before it is fed to the second contacting stage. That process involves a strong formation of very fine mist in the first and second cooling stages. That fine mist is not removed in the third cooling stage and can be removed only by mist collectors, which are very expensive and involve high operating costs. Valuable sensible gas heat from the contacting system is required to reheat the gases after the second cooling stage and an expensive acid cooler is required to cool the gases to a lower temperature in the first and second cooling stages.
It is an object of the invention to avoid the disadvantages of the known processes and to ensure particularly in the processing of gases having a substoichiometric ratio of H.sub.2 O to SO.sub.3 after the first contacting stage an optimum separation of the H.sub.2 O and SO.sub.3 contents while a formation of mist is avoided and an optimum utilization of the valuable heat of the gas from the contacting system is permitted.